DOI: 10.1051/forest:2003087Original article Relationships between hydraulic traits and habitat preference for six Acer species occurring in the French Alps Jérôme TISSIERa, Luc LAMBSb,
Trang 1DOI: 10.1051/forest:2003087
Original article
Relationships between hydraulic traits and habitat preference
for six Acer species occurring in the French Alps
Jérôme TISSIERa, Luc LAMBSb, Jean-Paul PELTIERa, Gérard MARIGOa*
a Laboratoire d’Écologie Alpine, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
b Centre d’Écologie des Systèmes Aquatiques Continentaux, 29 rue Marvig, 31055 Toulouse Cedex 04, France
(Received 23 October 2002; accepted 11 April 2003)
Abstract – Xylem hydraulic properties and vulnerability to cavitation were studied in six Acer species which occur, in the French Alps, along
a soil moisture gradient from the moist valleys to the drier mountain stands The results obtained suggest that a relationship existed between hydraulic properties and the species moisture preference Maples located in dry zones (A opalus, A monspessulanum) proved more resistant
to drought-induced cavitation than species that occurred in an irrigated area (A negundo, A pseudoplatanus, A platanoides, A campestre) The most vulnerable species to cavitation (A negundo, A pseudoplatanus, A platanoides, A campestre) showed the highest hydraulic conductances, whereas more cavitation-resistant species (A opalus, A monspessulanum) had the lowest conductances This suggests a
trade-off between hydraulic efficiency and cavitation vulnerability The possible ecological significances of these data are discussed in relation to the
distribution of Acer species in their natural habitats.
drought tolerance / hydraulic conductance / xylem cavitation / Acer species
Résumé – Étude des relations entre les caractéristiques hydrauliques et les exigences écologiques de six espèces d’Acer dans les Alpes françaises Les propriétés hydrauliques du xylème et la vulnérabilité à la cavitation ont été étudiées chez six espèces d’Acer qui se distribuent,
dans les Alpes françaises, selon un gradient d’exigence hydrique depuis les vallées humides jusqu’à des zones plus sèches de moyenne montagne Les résultats obtenus suggèrent l’existence d’une relation entre les propriétés hydrauliques et le gradient d’exigence hydrique des
différentes espèces d’Acer Les érables situés dans les zones sèches (A opalus, A monspessulanum) sont plus résistants à la cavitation que ceux implantés dans les stations bien alimentées en eau (A negundo, A pseudoplatanus, A platanoides, A campestre) De plus, les espèces les plus vulnérables à la cavitation (A negundo, A pseudoplatanus, A platanoides, A campestre) possèdent les conductances hydrauliques les plus élevées et inversement pour les espèces les moins sensibles (A opalus, A monspessulanum) Ceci suggère l’existence d’un « trade-off » entre
efficience hydraulique et vulnérabilité à la cavitation Ces résultats sont analysés au plan écologique en relation avec le mode de distribution des différentes espèces dans leur environnement respectif
tolérance à la sécheresse / conductance hydraulique / cavitation du xylème / Acer species
1 INTRODUCTION
Terrestrial plants depend a great deal on the transport of
water from the soil through the plant Water availability is one
of the most important factors which influence not only the
growth and development of plants, but also the spatial
distri-bution of species in their appropriate habitat [1, 11] Cyclic
droughts favour the establishment of species which are able to
acclimate to water deficit, and tend to eliminate species that
are not able to do so
There is ample evidence indicating that plant hydraulic
conductance is limited by drought and freezing stress, thus
restricting their water balance, their gas exchange, and their
growth [6, 30] An important component of hydraulic
architec-ture is the vulnerability to drought-induced xylem cavitation
Cavitation is the abrupt change from liquid water under ten-sion to water vapor resulting, when xylem tenten-sion exceeds a critical value, in the breakage of the water column [6, 22, 26,
28, 32] It has been suggested that xylem vulnerability to cav-itation may be the most important character determining drought tolerance in plants [30] Catastrophic xylem dysfunc-tion occurring through the entire xylem results in the death of the plant [29] Consequently, studying the differences in the hydraulic architecture, such as hydraulic conductance and vul-nerability to cavitation of plants, may help us to understand species habitat preferences with regard to water availability in soils
It had been suggested that there is a trade-off between the xylem hydraulic conductance and the xylem vulnerability to cavitation [32] This trade-off hypothesis was based on the
* Corresponding author: gerard.marigo@ujf-grenoble.fr
Trang 282 J Tissier et al.
assumption that wider conduits were more vulnerable to water
stress-induced cavitation This hypothesis remains
controver-sial since other studies have shown that vulnerability to
drought-induced cavitation is not related to conduit diameters
but rather to the structural properties of the interconduit pit
membranes [1, 21, 28] Thus both the existence of and lack of
a trade-off between xylem conductance and vulnerability to
cavitation have been documented
In this study, we concentrated on certain coexisting species
from the highly diverse genus Acer, with respect to their
spa-tial distribution in the French Alps These species are
distrib-uted along an ecological gradient from the moist alpine valleys
to the drier low mountains in relation to their water
consump-tion [23, 24] They were selected because they are
representa-tive of different levels of adaptation to drought, ranging from
species well adapted to drought such as Acer monspessulanum
L and Acer opalus Mill., to water-demanding species such as
A negundo L and A pseudoplatanus L Some properties of
the hydraulic system, the hydraulic conductance and the
vul-nerability to cavitation, were characterized to determine if
spe-cies with different habitat preferences had different hydraulic
characteristics, and also to see if differences in hydraulic
architecture between species might explain the habitat
prefer-ences The hypothesis was that species found in drier habitats
would be more resistant to water stress-induced cavitation
than species restricted to wet sites In conjunction with this
hypothesis, this study also attempts to verify, for this group of
closely related species, the existence (or not) of a trade-off
between xylem conductance and xylem vulnerability to
cavi-tation
2 MATERIALS AND METHODS
2.1 Study site and plant material
Measurements were made in June and July 2000 and 2001 on six
species of Acer growing in their natural habitat on two different sites.
The first site, which is located along the Isere river, on the Campus of
the University of Grenoble (45° 20’ N, 5° 30’ E, elevation 200 m), is
well-watered [17] A negundo, A pseudoplatanus, A platanoides L.,
A campestre L (10-15 years old, 10 m tall) occur in this place, mixed
with other co-existing tree species (Tilia cordata Mill.), on an alluvial
soil with a water table at a depth of between 2.20 and 2.50 m, on
aver-age Some experiments were also performed on a A monspessulanum
species which was cultivated at the arboretum of the Campus The
other site, known as “La Bastille”, is located in a low mountain stand
in the extern zone of the North-western Alps (45° 12’ N, 5° 44’ E,
ele-vation 800 m) This site is characterized by relative dry conditions
Vegetation, soil and climate have been described in detail by
Man-neville [16] In this area, the coarse texture (limestone bedrock
out-crops) explains the dryness of the soil and in summer, with the lack
of rainfall, this site is subjected, periodically, to drought [16] The
location was characterized by the presence of Acer opalus and Acer
monspessulanum, in association with Fraxinus excelsior L and
despite the unfavourable environmental dry conditions for its growth,
some trees of A pseudoplatanus were also present here However,
compared to the alluvial floodplains, the size of maple trees present
on the dry sites is smaller (4–6 m) All trees examined were
approxi-mately 12 years old In most of the experiments carried out in the
sta-tions, three to four trees per species were studied for each population
2.2 Hydraulic conductivity analysis
Xylem hydraulic efficiency was determined on 1-year-old twigs from branches collected with a pruning-scissors in the morning (between 7 and 8 h, solar time) from mature trees The branches were enclosed in black airtight plastic bags to reduce water loss through transpiration, and quickly brought to the laboratory for hydraulic anal-ysis In the laboratory, the branches were cut again under water After rehydratation, segments about 2–3 cm long were excised under water from different branches, shaved at both ends with a razor blade, and then fitted to plastic tubes at the basal end The segments were then perfused with filtered (0.2 µm) deionized water with a pressure dif-ference of 0.1 MPa for 10–15 min This treatment eliminated any air embolisms by successive water pressurization and restored the full capacity of the xylem [25] After removing the gas bubbles in the water, maximum conductivity (Kmax, mmol s–1m MPa–1) was deter-mined by forcing distilled water, with a pressure difference of 3.7 kPa, through each sample The resulting flow rate (mmol s–1) was measured using an analytical balance (Sartorius) At the end of the measurement, the segment diameter was measured (m, bark not included) to deter-mine the xylem-area-specific conductivity (Ks, mol s–1 MPa–1 m–1) which takes into account vessel diameter and the number of vessels
in the samples [12, 31]
2.3 Vulnerability curves
Vulnerability curves (VCs) were established for excised well-watered branches in which embolism was induced in a long pressure chamber (0.4 m), as described by Cochard et al [3] Air pressure in the chamber was maintained at the designated values (between 1 and
5 MPa) using nitrogen, until sap exudation ceased (after 10 to 60 min, depending on the pressure applied) For each pressure treatment, the percentage loss of hydraulic conductivity (PLC) was measured for 6
to 8 randomly selected stem segments The shape of the sigmọd curve was characterized by three critical points, , and which indicated the water potential values that induced the start of the embolism, 50% and 100% of the maximal hydraulic conductivity, respectively and were measured graphically from each VC Duplicate VCs were produced for two trees of each population
2.4 Statistical analysis
Analysis of variance (ANOVA) was used to compare hydraulic conductances among individuals of each species and to compare spe-cific conductances among species When necessary means of spespe-cific conductances were compared with Tuckey-Kramer test The analyses were performed with SPSS 9.0 (SPSS Inc, Chicago, IL)
3 RESULTS 3.1 Hydraulic characteristics
In all of the studied species (Fig 1), a linear relation was observed between the maximal hydraulic conductance (Kmax) and the diameter of the stem segments used as indicated by the strong coefficients of determination obtained for every indi-vidual The analysis of variance and the tests of Tuckey-Kramer indeed showed that the individuals were not signifi-cantly different among them
Three maples of Montpellier, taken from the xeric site (“La Bastille”, [16]) were compared with an individual from the arboretum of the campus where the ground is well supplied
with water Individuals of A pseudoplatanus present on both
ψcav ψ50 ψ100
ψcav ψ50
Trang 3studied sites were also compared (Fig 1) The analyses of the
variance showed, for both species, that the four individuals
were not significantly different
The values of specific conductivity, for the six species of
maple, are represented in Figure 2 On examination of these
data, two groups of species can be distinguished: a first group,
including A negundo, A pseudoplatanus, A platanoides and
A campestre, presents specific conductivities of the order of
23 mol s–1 MPa–1 m–1; a second group, including A opalus and
A monspessulanum, possess 2 times lower specific conductivities The statistical tests (ANOVA, P < 0.01) and
Tuckey-Kramer test show that the difference among species of both groups is highly significant Both aforesaid groups distinguish themselves thus very sharply by their specific conductivities
Figure 1 Xylem hydraulic conductivity (Kmax) as a function of branch diameter for maple trees Xylem segments 2 cm long were excised
from 1-year-old shoot internodes of adult branches For each species, these experiments were made on three to four individual trees depicted
on the graph by the different symbols In the case of A pseudoplatanus, the measurements were conducted on 3 trees originated from the wet
site (arboretum) together with the tree growing under unfavorable conditions in the low mountain stand (La Bastille, closed symbols) For
A monspessulanum, three individual trees in the mountain stand were compared with a tree of the wet site (closed symbols) The linear relation between Kmax and the stem diameter is given by the regression coefficient R2 Analysis of variance (ANOVA) was used to compare hydraulic conductances among individuals of each species
Trang 484 J Tissier et al.
The least water-demanding species, implanted on relatively
dry area (A opalus and A monspessulanum) have specific
conductivities less high than the species acclimated and
installed on grounds fed well with water (A campestre, A
pla-tanoides, A pseudoplatanus and A negundo).
3.2 Vulnerability to cavitation
The VCs of the six species of Maple are grouped together
in Figure 3 It is possible to differentiate, according to their
behavior, three groups individuals The first group comprises
A negundo, A pseudoplatanus, A platanoides and A camp-estre with vulnerable xylem (Ψcav near –1.2 MPa and Ψ100 between –2.25 and –2.75 MPa) The second group is
repre-sented by A monspessulanum which presents the strongest
resistance to the cavitation (Ψcav = –3 MPa; Ψ100 = –4.5 MPA)
A opalus showed an intermediate curve The major difference
with the first group was that Ψcav in A opalus was more negative
(Ψcav = –2 MPa) and the VC was steeper These data show that
when A monspessulanum and A opalus begin only to cavitate, the most sensitive species (A campestre, A platanoides, A pseudoplatanus and A negundo) are going to undergo 50 to
100 PLC The most resistant species qualify as xerophilic or
mesoxerophilic (A monspessulanum and A opalus) On the
other hand, vulnerable the species were those on sites with
mes-ohygric to mesic character (A negundo, A pseudoplatanus,
A platanoides and A campestre).
3.3 Hydraulic efficiency and vulnerability to cavitation
The combined results of the specific conductivities and the vulnerability to cavitation (Ψcav and Ψ50) for the six species of maple are represented in Figure 4 High hydraulic efficiency was related with vulnerable xylem The most vulnerable
spe-cies (A negundo, A pseudoplatanus, A platanoides and A campestre) were those which had the highest hydraulic effi-ciencies and the most resistant species (A monspessulanum and A opalus) had a low hydraulic conductivity indicating a
trade-off between hydraulic efficiency and vulnerability to cavitation
4 DISCUSSION
In woody plants, a general trend seems to be that there is a relationship between hydraulic traits and habitat preference This is true for example among desert plants, in Angiosperm and Coniferous trees [2, 8–10, 15, 30] The data presented in this study also provide evidence of the same relationship
exist-ing among coexistexist-ing species in the highly diverse genus Acer.
Figure 2 Comparison of the specific conductivity (Ks, mol s–1 MPa–1
m–1) for the six Acer species A ne: A negundo; A ps: A
pseudo-platanus; A pl: A platanoides; A ca: A campestre; A op: A opalus;
A m: A monspessulanum These experiments were carried out with
four individuals trees for each species Each value on the graph is the
mean of 30 to 40 measurements Vertical bars indicate the standard
error Values with different letters are significantly different
(ANOVA, P < 0.01).
Figure 3 Comparison of the vulnerability to embolism for six
spe-cies of the genus Acer These data are obtained from two individual
trees of each species, and each value on the graph is a mean of 6 to
8 replicates The standard errors, which did not exceed 5% of the
mean values, are not reported
Figure 4 Xylem efficiency versus xylem vulnerability to cavitation.
Xylem hydraulic efficiency is estimated by the specific hydraulic conductivity (Ks, mol s–1 MPa–1 m–1), and xylem vulnerability by the xylem water potential inducing the start of the embolism (Ψcav MPa, open symbols) and 50 PLC (Ψ50 MPa, black symbols)
Trang 5Comparison of curves representing vulnerability to cavitation
show that maple species situated naturally in dry zones (A.
monspessulanum and A opalus) have a greater resistance to
cavitation than those planted in sites with a good water supply
(A negundo, A pseudoplatanus, A platanoides, A
campes-tre) Furthermore, differences in specific conductivities also
appear between species: i.e A negundo, A pseudoplatanus,
A platanoides, A campestre have high specific conductivities
which are 2 times higher than A monspessulanum and A opalus.
A relationship, thus, appears to exist between the hydraulic
conductance and vulnerability to cavitation in the Acer
spe-cies
The hydraulic characteristics of branches seem to be very
specific to each maple species Indeed, when comparing
spe-cies in their natural habitat (wet sites for A pseudoplatanus,
dry sites for A monspessulanum) there is no modification in
hydraulic conductivity (Fig 1) or vulnerability to drought
cavita-tion (data not shown) in the A monspessulanum species
culti-vated at the arboretum of the campus, or in the A pseudoplatanus
species growing on the dry site of “La Bastille” This hydraulic
behavior can be related to that of Fraxinus excelsior, a species
that frequently occurs with maple trees Ash trees have a broad
ecological amplitude [18–20] in particular since these species
acclimate to limited water availability by increasing their
resistance to cavitation [13], one of the major causes of decline
in the botanical species in the absence of water [5] In contrast,
Acer species are more restricted to their habitats and do not
have this adaptation potential Therefore, the survival of
maples in a given biotope does not appear to be connected to
acclimation capacities but rather to specific hydraulic features
for each species While cavitation resistance seems to show
some phenotypic variability in ash, our study highlights
spe-cies-specific differences in this trait in maple In the case of
maples, the various species distinguish themselves according
to their hydraulic properties, the fruit of an ancestral evolution
in biotope preference, that as conferred on each a set of
acquired characteristics and ecological preferences marked for
a certain type of environment
This ecological preference is very marked according to wet
zone type: one comprehends poorly why A negundo, which
already begins to cavitate at hydric potential values lower than
–1 MPa, can develop in areas where the hydric constraints can
become very great This ecological preference is less
restric-tive for the species adapted to the dry zones So, by possessing
hydraulic properties of plants adapted to relatively dry
condi-tions, A monspessulaum, for example, managed to develop
without visible handicap in the arboretum of the campus (well
watered area)
As has been demonstrated for several other species [4, 7, 31],
our results also suggest a trade-off between xylem conductance
and vulnerability to drought-induced xylem cavitation in the six
species studied The maples species that resist to cavitation
pos-sess a low hydraulic conductance, while those that are more
vul-nerable to cavitation present a high hydraulic conductance The
significance of this trade-off should be investigated through the
study of structural/functional relationships The mechanism by
which xylem vulnerability acclimates to water stress is known
to depend directly on pit pore membrane diameter, whereas
hydraulic conductance is mainly related to conduit diameter
[14, 27, 32] During their development, the different tree
com-ponents acclimate to the prevailing environmental conditions, and therefore develop structures that acclimate them to envi-ronmental change Under wet conditions, plants optimize water conductance to accelerate their growth rates by developing conduits of larger diameter adapted for high water transport In contrast, plants need to invest less in means of acquiring water for their growth in dry habitats, and therefore reduced in xylem vulnerability and in hydraulic conductivity may be advanta-geous in order to avoid drought-induced embolism and water transport limitations These processes may be associated with small pores in the pit membranes and small diameters for water conducting vessel diameters However, it is likely that other factors, including tracheid length or number and area of pits per tracheid wall, also influence xylem hydraulic properties in a complex way
Acknowledgements: This work was supported by financial
assist-ance from the European Community, Contract No
EVK1-CT-1999-00031 (Proposal N° EVK1-1999-00154 Flobar 2) The authors thank Nadia Barsoum, on leave from the University of Cambridge, for cor-recting the English, and J.P Guichard for technical help
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